This invention relates to capacitive transducers, for example for use in displacement and force-responsive devices.
Capacitive displacement transducers are known for use in displacement and force-responsive devices such as measurement probes and joysticks, where a stylus or lever is movable in the directions of two or more orthogonal axes. Normally there would be one or more separate capacitive transducers for each axis of movement, as shown in U.S. Pat. No. 5,661,235. This not only results in a costly, difficult to assemble structure, but also degrades the performance when measuring extremely small forces and displacements, due to the large moving mass which limits the measurement frequency response and increases the sensitivity to external vibrations. Thomas (U.S. Pat. No. 5,006,952) teaches a multiple axis capacitive displacement transducer that uses a single movable pick-up plate. This device does not have the difficult assembly and large moving mass problems as the individual transducer devices have, but it has two other drawbacks, when applied to high precision measurements, that are solved by the present invention. The first drawback is that the pick-up plate is supported, and pivots about a point on the stylus or stem some distance away from the pickup plate. This causes an undesired translational motion in addition to the desired rotational response for x and y axis displacement of the stem. This undesired translational response may generate undesired cross-axis readings. For instance, a pure x-axis displacement of the stem may produce a false Z-axis reading. The other problem is that the signal channels are separated from each other by operating at different frequencies. This results in the individual channels having different noise and frequency response characteristics, which is generally undesirable for precision measurements.
Precision capacitive displacement transducers typically employ three electrodes, which form a structure equivalent to two capacitors connected in series, with the center electrode being movable and common to both capacitors. The center electrode is also typically the pick-up electrode, and the two outer electrodes are mechanically fixed. Although the transducer is fundamentally responsive to displacement of the center electrode, it can be used to measure force, by the deflection of springs of known stiffness in response to that force, as well as acceleration or pressure. Bonin et al. (U.S. Pat. No. 4,694,687) discloses a vehicle performance analyzer which incorporates a capacitive accelerometer based on the three electrode structure described here. By driving the outer electrodes with two equal amplitude signals 180 degrees out of phase, the voltage on the center electrode is a linear function of the displacement from the center, with the phase giving polarity information. The full scale amplitude of the output signal is equal to the amplitude of the drive signal. This is a great improvement over strain gauge type load cells which are also used to measure displacement and force. Strain gauges typically have a full scale output signal that is 0.2% of the input signal, giving the capacitive transducer 500 times greater output signal.
By synchronously demodulating the center electrode signal of the capacitive transducer, a DC voltage proportional to the displacement is generated. In the absence of parasitic effects such as amplifier input capacitance, the output signal of voltage vs. displacement would be perfectly linear, but for conveniently scaled devices, the transducer source capacitance may be on the order of 5 pF and the parasitic capacitance may be 1 pF or more, so the non-linearity is significant, on the order of 20% at full scale in this case. For multiple axis devices sharing a common center electrode, the parasitic capacitance may actually be greater than the sense capacitance per axis. It is possible to eliminate the effect of this parasitic capacitance on the signal linearity by generating a feedback signal that is added to one drive signal and subtracted from the other, in order to maintain a null, or zero voltage situation on the center electrode regardless of displacement. In this case the feedback signal is used as the output signal, and is proportional to the center electrode displacement regardless of parasitic capacitance. Thomas uses this feedback method, and also references British Patent No. GB 1366284.
One application requiring measurement of force and possibly displacement in at least two directions is scratch testing of materials to determine coating adhesion and resistance to wear. In this test, a series of passes at increasing loads are made over the material with a stylus, until reaching a load that causes de-lamination or other catastrophic failure. Typically, both the vertical and horizontal load forces are recorded vs the horizontal position. The vertical displacement of the stylus into the sample may also be recorded. A more sophisticated test uses a ramped vertical load to get the same information from a single, rather than a series of scratches. This measurement requires that there be very little interaction between the signals of the different axes, so that the coefficient of friction, that is the horizontal force divided by the lateral force, can be accurately determined. A similar, more specific application involves tribological studies of materials for rigid disc drive applications, such as measuring the friction properties of various slider materials on a disc surface. In both cases, a low moving mass is desirable to allow a higher measurement bandwidth than is possible with prior art devices such as described in U.S. Pat. No. 5,661,235.
Another application requiring high measurement sensitivity due to the small size of the devices being measured is in the mechanical testing of MEMS devices. These devices are typically 100 to 1000 microns in length, and may have elements with dimensions as small as 1 micron in width or thickness. Due to the very small size of the parts, force sensitivity of one micro newton or better is desired. Multiple axis capability is desired so that measurements can be made in any direction required by the sample, although each measurement is typically made in a single direction.